Lubricants undergo physical changes during operation due to oxidation of the lubricants. Typically, oxidation results from high temperatures, the presence of oxygen dissolved in the lubricant itself as well as mixing of the lubricant with oxygen supplied from the air. Among the problems associated with oxidation of lubricants are varnish formation on the pistons, ring sticking due to formation of carbon deposits and increased bearing corrosion due to formation of acids. Salt formation results from dissolved metals which together with oxidized lubricant form a sludge leading to increased viscosity of the lubricant. All of the foregoing can lead to increased fuel consumption and serious engine damage. Oxidation inhibitors have been developed to alleviate these problems.
Mannich base reaction products of mono-alkyl-substituted phenols, aldehydes and amines have been described as detergent-antiscuff additives in lubricants for 2-stroke gasoline engines in U.S. Pat. No. 4,025,316 to Stover and as detergent and antirust additives in hydrocarbon combustion fuels in U.S. Pat. No. 3,649,229 to Otto.
The Mannich base reaction products of alkenylsuccinic anhydrides, aryl-substituted monoamines and alcohols have been described as antioxidants in lubricants in U.S Pat. No. 4,803,004 to Andress et al.